1. Technical Field
The present disclosure relates to the use of power transistors driving an electric current to preheat a motor stator before using the motor, and in particular, to apparatus and methods for increasing the lifetime of such power transistors when they are used during stator heating.
2. Background of Related Art
There are many reasons why an electric motor may require preheating before it is started. For example, if a motor is at relatively low temperatures, the viscosity of the grease in the motor's bearings may not provide suitable lubrication, leading to bearing wear. Additionally, sub-zero temperatures may lead to icing between the motor's rotor and stator, inhibiting the motor's ability to start, or leading to damage.
In addition, it may be advantageous to preheat motors driving compressors in air conditioning or heat pump systems, because refrigerant tends to migrate towards the coldest location. Typically the compressor motor is outdoors and exposed to cold air during winter, whereas equipment indoors is kept at a reasonably warm temperature, so refrigerant will migrate and condense into a liquid form inside the shell of an outdoor compressor and the coil of the outdoor unit. If, at that moment the compressor is started, the compressor will be effectively trying to compress liquid, which can lead to increased compressor wear, damage, and/or failure. Compressor heating devices, such as bellyband heaters or a preheat mode, are typically used to warm up the interior or shell of the compressor. By keeping the compressor warm, any refrigerant in that area will be kept in a gaseous phase, and liquid refrigerant will migrate somewhere else, thus improving the total life of the compressor.
A preheat mode of an idle compressor generates heat within the windings of the idle compressor motor to a given motor temperature range, and can be controlled by sensing the motor temperature with a suitable sensor, such as a thermistor. A preheating mode of the idle compressor is cycled on for a given time period (e.g., 15 minutes), and can then be cycled off for a given delay period (e.g., 30 minutes) before the preheating time period is reactivated, thereby providing an optional failsafe time functionality.
Modern variable speed drive systems have been employed to run synchronous permanent magnet rotor motors used in compressors. A typical schematic diagram for a prior art three phase drive system is illustrated in FIG. 1. One technique for preheating stator windings of a motor before running the motor includes turning on a power supply transistors, such as insulated gate bipolar transistor (IGBT), in the drive to supply current to a stator winding of the motor. A standard procedure is to select one output transistor and two return transistors for stator heating current, and to continually use only these transistors for stator heating, during the entire lifetime of the system. For example (with reference to FIG. 1), by turning on three transistors in the drive (having gate terminals identified as T1, T5 and T6), nearly 30 Amperes would be applied to the stator winding of a compressor motor in a 5-ton system, due to low impedance of the stator windings. The entire 30 Ampere current would run through one IGBT (T1), pass through the U-phase winding, and then return 15 Amperes via the V- and W-phases and the associated IGBTs (T5 & T6) respectively to the power source, as illustrated in FIG. 1. Over the lifespan of the drive, the large 30 Ampere current passing through the IGBT associated with the U-phase transistor can significantly degrade this single IGBT device.
A motor drive system which improves the overall lifespan and reliability of drive components such as IGBTs would be a welcome advance in the art.